Plastics fabrication normally involves the use of massive machinery to extrude, mold, and calendar very viscous molten polymer. Due to the very high viscosity of such polymer melts, high pressures are used in the molding or extrusion process, often exceeding thousands of pounds per square inch. In addition to the substantial investment in the machinery for such processes, there are important limitations on the shape of the plastic object that can be generated in such operations. For example, it is often difficult to make extremely thin coatings or films of a plastic through an extrusion operation because very high temperatures are often required to generate the low melt viscosities needed for such thin objects. Often the polymers cannot withstand the high temperatures needed to provide such viscosities, and therefore, dilute solutions of such polymers in various organic solvents are employed to cast coatings. If this latter technique is employed (solvent casting) many disadvantages are encountered. Such a process involves substantial cost due to solvent recovery and the relatively low throughput of polymer per unit volume of intermediate (polymer plus diluent) handled. These and many other disadvantages make the fabrication of polymers an expensive process and often preclude fabrication of certain intermediate shapes.
Various techniques have been developed to circumvent these difficulties in polymer fabrication. Some involved a casting operation in which reactive prepolymers are combined with catalyst and coreactants, and poured as viscous liquids into the proper receptacle. Under these conditions a chain lengthening reaction ensues which is expedited by conducting the process at elevated temperatures. Such reactions often involve crosslinking steps in order to provide proper dimensional stability as rapidly as possible. Ideally, one might hope to conduct the polymerization process directly from monomer to yield the final product in desired form. Unfortunately the polymerization of most monomers is accompanied by the evolution of substantial amounts of heat and substantial shrinkage. This presents a major obstacle in creating plastic objects with precise surface detail in any casting operation involving significant amounts of low molecular weight monomer.